RoutingError when enabling Assets-list.

[aapo]

My setup:

redmine: 2.3.1-1
ruby-rails: 3.2.13-2
ruby: 1.9.3.194-8.1ubuntu2 
ruby-rack: 1.4.5-1
apache2: 2.4.6-2ubuntu2
libapache2-mod-passenger: 3.0.13debian-1.2

My Redmine installation works with apache2. Installing redmine-assets-plugin
works. I can see it in modules-list, but enabling assets causes error:

ActionController::RoutingError (No route matches {:project_id=>#<Project id: 1, name: "Test", description: "", homepage: "", is_public: true, parent_id: nil, created_on: "2013-11-28 10:26:16", updated_on: "2013-11-28 10:26:16", identifier: "test", status: 1, lft: 1, rgt: 2, inherit_members: false>, :controller=>"assets"}):
  lib/redmine/menu_manager.rb:150:in `render_single_menu_node'
  lib/redmine/menu_manager.rb:102:in `render_menu_node'
  lib/redmine/menu_manager.rb:91:in `block in render_menu'
  lib/redmine/menu_manager.rb:168:in `block in menu_items_for'
  lib/redmine/menu_manager.rb:165:in `each'
  lib/redmine/menu_manager.rb:165:in `menu_items_for'
  lib/redmine/menu_manager.rb:90:in `render_menu'
  lib/redmine/menu_manager.rb:80:in `render_main_menu'
  app/views/layouts/base.html.erb:45:in    `_app_views_layouts_base_html_erb___2841094345632406335_31656900'
  app/controllers/projects_controller.rb:163:in `show'